1. Field of the Invention
The present invention relates to the removal of subsurface contaminants using underground heating by steam injection and application of electric currents. In particular, the invention is a method for removing volatile organic contaminants from heterogeneous soils and rock by vacuum extraction and pumping of fluids heated by steam and electric currents.
2. Description of Related Art
Concentrated underground organic contaminant plumes, such as from a leaking underground storage tank, are one of the most prevalent groundwater contamination sources. If the stored liquid escapes from the tank slowly, the operator may not become aware of the problem for years. By that time, the solvent or fuel can percolate deep into the earth, penetrating permeable layers of sand and gravel and relatively impermeable clays, and often migrating to water-bearing regions.
These contaminants collect underground as a separate, liquid organic phase called non-aqueous-phase liquids (NAPLs) and continually compromise the surrounding groundwater. This type of spill is one of the most difficult environmental problems to remediate. Removing the contaminants by pumping the groundwater requires a huge amount of water to be washed through the contaminated area and can take decades. This method has been used at some sites, and although pumping over many years resulted in clean effluent water, the groundwater became contaminated again when the pumps were shut off and restarted years later.
Another method of remediation is to heat the subsurface to vaporize the underground contaminants, and then remove them by vacuum extraction. Many methods have been proposed for underground heating. The principal technology is steam injection coupled with vacuum extraction, which was developed at the University of California, Berkeley. (See U.S. Pat. No. 5,018,576 by Udell et al.) This technique provides an efficient way to heat the subsurface, as well as a controllable sweeping mechanism to move and extract the contaminants. The major disadvantage of steam injection is that the steam does not penetrate clay or other low permeability layers well, leaving significant areas of the contaminated plume untreated. An additional mechanism is required to dry and clean the impermeable layers that are common in the soils of the western United States.
A method is needed that can be applied to heterogeneous soil and rock layers, which have highly variable fluid and gas permeabilities. In a heterogeneous site, the more permeable clay-rich units are generally more electrically conductive than the hydrologically permeable units, which are penetrated by the steam. This invention proposes to combine steam injection with another technique that can reach the areas impermeable to steam and thereby uniformly heat the entire contaminated area.
The most efficient and controllable method for heating clay or other low permeability layers on a large scale is direct electrical resistance heating. The clay layers act as the heating element when large electric currents are driven through them. The electrical resistance heating targets the clay-rich layers that are not well-penetrated by steam injection. Electric heating is self-limiting, in that the electric current will stop flowing as the clays heat up and dry out.